Composite structures are used in a wide variety of applications due to their high strength-to-weight ratio, corrosion resistance, and other favorable properties. In aircraft construction, composites are used in increasing quantities to form the fuselage, wings, horizontal and vertical stabilizer, and other components. For example, the horizontal stabilizer of an aircraft may be formed of composite skin panels co-bonded to internal composite structures such as composite stiffeners or spars. The composite spars may extend from the root to the tip of the horizontal stabilizer and may generally taper in thickness along a spanwise direction to improve the stiffness characteristics of the horizontal stabilizer and reduce weight. Composite spars may also include localized increases in the composite thickness such as where the spar attaches to other structures or components.
Composite stiffeners or spars may be provided in a variety of cross-sectional shapes. For example, a composite spar or stiffener may be formed in an I-beam shape by bonding together the vertical webs of two “C” composite channels in back-to-back arrangement. Each one of the “C” channels may have horizontal flanges extending outwardly from the upper and lower ends of the web. Each horizontal flange may transition into the web at a radiused web-flange transition. When the “C” channels are joined back-to-back to form the I-beam shape, the radiused web-flange transitions result in a lengthwise notch along the upper and lower ends of the I-beam. The lengthwise notches may be referred to as radius filler regions or noodle regions. To improve the strength, stiffness, and durability of a composite structure, radius filler regions may be filled with radius fillers or noodles formed of composite material.
Unfortunately, existing radius fillers suffer from several drawbacks that detract from their utility. For example, existing radius fillers may exhibit cracking due to residual stress that may occur during the manufacturing process. In addition, certain radius fillers may result in relatively low pull-off strength at the bond between the I-beam and the skin panel. Furthermore, radius fillers formed of unidirectional fibers may prevent non-destructive inspection (NDI) of the inner radii at the web-flange transitions in the I-beam. For example, the geometry of the radius fillers may prevent inspection of the inner radii using conventional acoustic inspection methods due to adverse effects on acoustic transmissions. Further in this regard, localized changes in the composite thickness along the length of the I-beam may result in variations in the contour of the inner radii of the I-beam which may complicate acoustic inspection of the inner radii.
As can be seen, there exists a need in the art for a radius filler that minimizes cracking during the composite manufacturing process and which provides favorable pull-off strength. Furthermore, there exists a need in the art for a radius filler that improves the inspectability of the inner radius of a composite structure. Preferably, the radius filler can be manufactured in a low-cost and relatively rapid manner with a minimal amount of touch labor.